The production of maleic anhydride through the catalytic oxidation of a hydrocarbon in a vapor phase is well known. Although benzene and air have conventionally been used as starting materials for the oxidation reaction in the presence of a vanadium pentoxide catalyst in the above process, techniques have recently been developed in which a chain hydrocarbon having 4 carbon atoms, e.g., butane, butene, or butadiene, is used as a starting material. The mainstream technique among these is to use n-butane, which is a saturated hydrocarbon, as a starting material and conduct the oxidation reaction thereof in the presence of a catalyst comprising a vanadium-phosphorus mixed oxide as the active component.
The maleic anhydride contained in the yielded reaction mixture gas is recovered, for example, by cooling the reaction mixture gas to condense the maleic anhydride, or by bringing the reaction mixture gas into contact with water to collect the maleic anhydride as maleic acid in the water. However, these recovery techniques each has problems inherent therein.
The technique for recovering maleic anhydride which is currently thought to be the most preferred is to bring a reaction mixture gas into contact with an organic solvent to collect maleic anhydride in the organic solvent. For example, in U.S. Pat. No. 4,118,403 is disclosed a technique in which a reaction mixture gas is contacted in an absorption column with a dialkyl phthalate in which each alkyl group has 2 to 8 carbon atoms to thereby collect maleic anhydride. The dialkyl phthalate containing maleic anhydride absorbed therein is stripped or distilled to recover the maleic anhydride therefrom, and the residual dialkyl phthalate is cooled and then circulated to the absorption tower. In JP-B-1-25747 (the term "JP-B" as used herein means an "examined Japanese patent publication") is disclosed the use of a dialkyl ester of a hydrogenated phthalic acid, such as tetrahydrophthalic acid or hexahydrophthalic acid, in place of a dialkyl phthalate.
One of the problems of the techniques in which an organic solvent is circulated to recover maleic anhydride is that by-products of the reaction such as acrylic acid, maleic acid, fumaric acid, and high-boiling components are collected in the organic solvent and accumulate in the apparatus to plug or soil the apparatus. In the case where a phthalic ester or the like is used as the organic solvent, phthalic acid or the like is yielded by solvent decomposition and arouses the same problem. These accumulated components further accelerate decomposition of the solvent and, as a result, pose a problem that the solvent consumption per unit production of maleic anhydride increases and the production cost of maleic anhydride increases. Expedients which have been proposed for eliminating these problems include a technique in which filtration and distillation are conducted (see U.S. Pat. No. 4,118,403) and a technique of washing a circulating organic solvent with water (see European Patent 0459543). However, even with these techniques, the problems caused by the accumulation of impurities in an organic solvent are difficult to mitigate to a satisfactory level.
It is known that in the case of using a catalyst comprising a vanadium-phosphorus mixed oxide as the active component, an effective method for maintaining the catalytic activity is to feed a phosphorus compound to the reactor (see, for example, JP-A-59-205373 and JP-A-60-143832 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")). It is thought that since phosphorus as one of the constituent elements of the catalyst vaporizes off during the reaction, the addition of a phosphorus compound in such a manner as to compensate for the vaporization loss can maintain the catalytic performance at a constant level. Consequently, when the reaction mixture gas, which contains phosphorus compounds resulting from vaporization from the catalyst, is contacted with an organic solvent, these phosphorus compounds also are collected in the organic solvent together with maleic anhydride. As a result of investigations made by the inventors of the present invention, it has been found that an organic solvent containing these phosphorus compounds is more susceptible to decomposition than the same organic solvent not containing these phosphorus compounds, and that these phosphorus compounds are difficult to remove by mere washing with water.